This invention relates to magnetrons and more particularly, but not exclusively to magnetrons operating at high power levels.
In one known magnetron design, a central cylindrical cathode is surrounded by an anode structure which typically comprises a conductive cylinder supporting a plurality of anode vanes extensive inwardly from its interior surface. During operation, a magnetic field is applied in a direction parallel to the longitudinal axis of the cylindrical structure and, in combination with the electrical field between the cathode and anode acts on electrons emitted by the cathode, resulting in resonances occurring and the generation of r.f. energy. A magnetron is capable of supporting several modes of oscillation depending on coupling between the cavities defined by the anode vanes, giving variations in the output frequency and power. One technique which is used to constrain a magnetron to a particular operating mode is that of strapping. To obtain and maintain the xcfx80 mode of operation, which is usually the mode which is required, alternate anode vanes are connected together by straps. Typically, two straps are located at each end of the anode or in another arrangement, for example, there may be three straps at one end of the anode and none at the other.
In another approach for selecting the mode of oscillation, the magnetron is designed such that the frequency of the xcfx801 mode is below cut-off. The magnetron is taken through the cut-off level very quickly so that there is insufficient power generated in the unwanted mode to produce significant oscillation which would otherwise result in power loss from the mode.
However, in some magnetrons, oscillations may occur simultaneously in the desired xcfx80 mode and also in the unwanted xcfx80xe2x88x921 mode despite the use of strapping, resulting in frequency instability and power being lost from the xcfx80 mode to the xcfx80xe2x88x921.
The invention is particularly applicable to magnetrons operating at high power levels, at 1MW or greater, and to magnetrons having a long anode in which it is difficult to achieve the required mode separation. The invention may also be advantageously used in other magnetrons not having these features.
According to the invention, there is provided a magnetron comprising: an anode having resonant cavities and coaxially arranged with a cathode about a longitudinal axis; output means including a coaxial line configured to receive energy in one oscillator mode and transmit it as a coaxial transmission mode and to receive energy in another oscillator mode and transmit it as a cylindrical waveguide mode; and means for at least reducing onward transmission of energy in the cylindrical waveguide mode.
Use of the invention enables energy in the undesired oscillator mode to be removed from the resonant cavities in addition to the energy in the desired mode and subsequently separated from the desired mode energy. Thus, power in the unwanted oscillator mode within the magnetron is reduced, tending to enhance operation in the desired mode and improving frequency stability and power output. The invention is particularly advantageously applied where the anode is long, for example, where the anode has an axial length of greater than hall wavelength (xcex/2) where xcex is the operating wavelength. For such long anodes, conventional strapping at the ends of the anode may be ineffective in maintaining the required mode separation. In addition, because a long anode allows high power levels to be achieved, in the absence of the invention significant amounts of energy would exist in the unwanted oscillator mode reducing power output in the wanted mode.
The invention may be advantageously employed in magnetrons of different designs, for example, the anode need not be of the vane type. Preferably, power is coupled from the magnetron in an axial direction. This gives a symmetrical output. In one arrangement, a cylindrical wall is located at the end of the anode and fingers are extensive between the wall and alternate anode vanes, to permit the xcfx80 mode to be extracted.
Advantageously, the coaxial line has at least one axially extensive slot through its outer conductor via which energy in the cylindrical waveguide mode is coupled from the coaxial line. In a coaxial transmission mode, the voltage is radial and the current travels in an axial direction whereas in a cylindrical waveguide mode, the currents are circumferential. Thus, the use of an axially extensive slot will not interfere with power transmission in the coaxial waveguide mode but will intercept current in the cylindrical waveguide mode. Advantageously, radiation absorbing material is located at the at least one slot to absorb energy radiated by the slot. Only one slot may be provided but it has been found that four slots located equidistantly around the outer conductor and located at the same position along the axis give particularly good performance. In one embodiment, the absorbing material is porous alumina impregnated with carbon. Longer slots tend to give greater energy absorption and a larger mass of absorbing material may be used to give greater capacity for absorption.
Preferably, the one oscillator mode is the xcfx80 mode and another oscillator mode is the xcfx80xe2x88x921 mode. Also it is preferred that the coaxial transmission is the TEM mode and the cylindrical waveguide mode is the TE11 mode The dimensions of the coaxial line are selected such that it supports both of these waveguide modes. For the TE11 mode, the cut off wavelength is equal to multiplied by the sum of the inner conductor diameter and the inner diameter of the outer conductor, the cut off wavelength being equal to or greater than the free space wavelength.
In an advantageous embodiment, there is included at least one axially extensive reflector slit in the output means for reflecting energy from said another oscillator mode back towards the resonant cavity. Thus energy in the cylindrical waveguide mode is coupled back to the resonant cavities. The reflector slits have no effect on the mode as it is transmitted in the TEM mode in which the currents flow axially. However, the xcfx80xe2x88x921 mode couples to the coaxial line in the TE11 mode having circumferential currents which are affected by the reflector slit or slits. By appropriately selecting the length and location of the slits, some of the TE 11 mode is reflected in a reverse direction along the coaxial line at a phase and magnitude determined by the slit geometries, increasing its coupling to the xcfx80xe2x88x921 mode in the anode. This gives increased loading, of the xcfx80xe2x88x921 mode, resulting in more stable operation of the magnetron, permitting it to operate over a wider range of input conditions and to be more tolerant of output and input conditions.
The reflector slit or slits may be in the outer conductor of the coaxial line, the inner conductor or in both. Where the slits are in the inner conductor of the coaxial line, in one preferred arrangement, the slit is extensive through the inner conductor, that is, it extends from one surface to the other. Advantageously, there are two reflector slits in the inner conductor which are both extensive therethrough and which intercept each other. In one embodiment, a reflector slit or slits may he located such that they are located partially or wholly in a region between the resonant cavities and the end of the coaxial line nearest the anode.
A magnetron in accordance with the invention may include a waveguide to which the coaxial line is arranged to deliver energy. The coaxial line may terminate in a T probe although alternative types of termination may be suitable.
Preferably, the coaxial line includes a discontinuity which at least reduces transmission along the coaxial line of energy reflected from the waveguide back towards the anode in a cylindrical mode. Thus, the coaxial line is dimensioned along its length to support both coaxial transmission and cylindrical waveguide modes, but its dimensions change at the termination so as to block transmission in the reverse direction of energy in the cylindrical waveguide mode.
In one magnetron in accordance with the invention, the coaxial line is designed such that both the TEM and the TE11 modes, say, can coexist. If the transition from the coaxial line to the waveguide is not perfect, some of the TEM power is reflected by the transition and, due to the transition""s asymmetrical shape, is converted into tile TE11 mode and transmitted in the reverse direction back towards the magnetron anode along the coaxial line. In a magnetron in which energy absorbing material is arranged to intercept power in the cylindrical mode, reflected output power might also be absorbed in the attenuator material causing the material to heat up and reducing overall efficiency of the magnetron. However, the inclusion of a discontinuity prevents power in the cylindrical mode being transmitted in reverse direction along the coaxial line as it is re-reflected at the discontinuity and transmitted along the output in a forwards direction. Preferably the discontinuity is located between the radiation absorbing material and the transition. Thus, the absorbing material is prevented from being heated by the output power of the magnetron to such an extent that it may give off gas and potentially destroy or reduce the life of the magnetron.
The invention is particularly advantageous for use with high power magnetrons, for example an X-band linac magnetron.
One way in which the invention may be performed is now described by way of example with reference to the accompanying drawings in which: